Spatiospectral lobe characteristics for F404 engine noise

Abstract

Spatiospectral lobes are a currently unexplained phenomenon seen in noise radiation from tactical aircraft. These lobes can be seen as either multiple peaks in the noise spectra at a given field location or multiple local maxima in the directivity when observing the radiated noise field at a particular frequency. Using hybrid beamforming of a 120-microphone near-field array, the characteristics of these lobes are studied for a GE F404 engine installed on a T-7A aircraft and at different engine conditions. Both the measured field and field reconstructions show multiple spatiospectral lobes. While the overall directivity of the noise shifts forward with increase in frequency, individual lobes appear, shift aft, and then disappear as frequency is increased. Individual lobes were also raytraced back to the jet centerline to determine an apparent acoustic source location. For frequencies where multiple lobes are present, they generally radiate from an overlapping maximum source region that moves upstream with frequency. This corroborates the characteristics of spatiospectral lobes observed in other aircraft. [Work supported by ONR.]